1. Field of the Invention
This invention relates to a composition for cross-linking ethylene.polymer which enables an ethylene.polymer to be cross-linked by heating while simultaneously retarding scorch and enhancing the degree of cross-linking, and to a method for cross-linking ethylene.polymer.
This invention further relates to a composition for the production of a cross-linked ethylene.polymer, which composition retains a liquid state at normal room temperature, exhibits only slight volatility, and exhibits high aging stability.
2. Description of the Prior Art
A well-known problem in the production of shaped articles of a cross-linked ethylene.polymer is that the compound suffers scorching in the molding device to such an extent that the appearance of the molded article is degraded, the amount of the compound extruded becomes irregular, and the products come to lack uniformity.
A number of methods are already known for preventing the compound from scorching in the molding device. For example, Japanese Patent Publication SHO 54(1979)-8500 discloses a method for preventing scorch by addition of a selected vinyl compound such as, for example, .alpha.-methylstyrene or an acrylic ester. U.S. Pat. No. 3,335,124 discloses a method for preventing scorch by addition of a sulfur compound. U.S. Pat. No. 3,202,648 discloses one based on the addition of a nitrogen compound, and Japanese Patent Publication SHO 47(1972)-36866 discloses one based on the addition of a phenol type compound.
Dicumylperoxide (hereinafter referred to as "DCP") is commonly used as a cross-linking agent for polyethylene. Since this compound has a melting point of 38.degree. C. to 40.degree. C. and is in a solid state at room temperature, it is liable to be contaminated with foreign matter during handling. This foreign matter can not be detected. Mixing of ethylene.polymer with the DCP is generally carried out using an extruder or the like. For the DCP to be supplied at a fixed speed, it has to be melted by heating. The use of DCP therefore adds to the number of process steps and requires measures for operational safety.
For the solution of this problem, it has been proposed to use for cross-linking an organic peroxide which is in a liquid state at room temperature. Japanese Patent Publication HEI 2(1990)-031106 discloses an organic peroxide mixture consisting of 25 to 10% by weight of bis(.alpha.-t-butylperoxyisopropyl)benzene and 75 to 90% by weight of isopropylcumyl-t-butylperoxy and Japanese Patent Public Disclosure SHO 54(1979)-132644 discloses a DCP substituted with an alkyl group.
Joint use of an organic peroxide and 2,4-diphenyl-4-methyl-1-pentene (hereinafter referred to as "MSD") in the curing of an unsaturated polyester resin has been disclosed (Reinforced Plastics, Vol. 29, No. 8, pp. 357 to 362), as has the joint use of an organic peroxide and MSD for the purpose of lowering the molecular weight of a polymer resulting from the polymerization of a vinyl monomer such as styrene (Japanese Patent Publication SHO 55(1980)-6895).
The method of Japanese Patent Publication SHO 54(1979)-8500 is effective for retarding scorching but has little or no effect toward enhancing the degree of cross-linking. .alpha.-methylstyrene and acrylic ester are low molecular weights and, therefore, are susceptible of volatilization. They are volatilized, therefore, while they are being mixed with the ethylene.polymer or while the composition produced by the mixing is being stored and are degraded in their ability to retard scorching and are apt to cause inconsistency in the quality of the cross-linked polymer.
The methods of U.S. Pat. No. 3,335,124, U.S. Pat. No. 3,202,648, and Japanese Patent Publication SHO 47(1972)-36866 are useful for retarding scorching. However, these methods have a disadvantage of slightly lowering the cross-linking degree. Further, the compounds used in these methods have toxicity.
As regards the organic peroxides which are in the liquid state at normal room temperature, it is preferable during winter to use an organic peroxide having a lower melting point than the organic peroxides disclosed in Japanese Patent Publication HEI 2(1990)-31106 and Japanese Patent Public Disclosure SHO 54(1979)-132644. Furthermore the organic peroxides which are liquid at room temperature and have a low vapor pressure are required. Further, it is required that such liquid peroxides don't suffer yellowing and lowering of active oxygen during storage.
The purpose of the using MSD during the curing of an unsaturated polyester is to delay the curing time and lower the temperature of generated heat. The heat generation temperature is governed by the polymerization conversion rate. It is evident, therefore, that MSD only functions to inhibit the polymerization. In the polymerization of styrene, for example, MSD is used as a chain transfer agent for the purpose of lowering the molecular weight.